There has recently been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. The development of secondary batteries capable of repeatedly charging and discharging has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available. Lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire or explosion, encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. However, additional research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, safety regulations strictly restrict the possibilities of dangers (such as fire and smoke) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo extreme thermal shrinkage at a temperature of 100° C. or higher due to their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause short circuits between a cathode and an anode.
Various proposals have been made to solve the above safety problems of electrochemical devices. For example, Korean Unexamined Patent Publication No. 2007-0019958 discloses a composite separator including a porous substrate and a porous coating layer coated on at least one surface of the porous substrate wherein the porous coating layer is composed of a mixture of inorganic particles and a binder polymer. The inorganic particles present in the porous coating layer coated on the porous substrate serve as spacers that can maintain a physical shape of the porous coating layer to inhibit the porous substrate from thermal shrinkage when an electrochemical device overheats. Interstitial volumes present between the inorganic particles form fine pores of the porous coating layer.
The composite separator is interposed between a cathode and an anode to fabricate an electrochemical device. The capacity of the electrochemical device tends to deteriorate to a considerable extent with increasing numbers of cycles.